Is it possible to modify the board/pieces?

Kind of wonder, is it possible? I've been experimenting with matching boards and backgrounds, and I wonder if that is possible as well.

Specifically, I mean with custom colors/textures... through modifying css or something, basically.

Yes, it is possible:
userstyles.org/styles/browse/lichess

Yes
My lichess theme

chessboards
http://gorgonian.weebly.com/lichessorg.html